In general punch press work, a plurality of replacement punch tools are used to expand machining conditions of the punch press machine. In addition, the punch press machine is arranged to simultaneously machine a plurality of products in the same punch press shape from a sheet of work to raise the machining efficiency of the punch press machine since, particularly, in the case of a small product, a lot of time is required to accurately position the work and feed it to the punch press machine. This is generally called a multiple product machining function. For carrying out multiple product machining by controlling a punch press machine with a numerical controller (NC unit), it is usual to prepare a machining program consisting of a plurality of instructions for machining a sheet of product (product sheet) and instructions to be executed as many times as specified while changing the reference points specified in the above machining program, and store these program and instructions in advance in a memory. When the NC unit is commanded to execute these instructions, the NC unit accesses the machining program as many times as commanded in the program. When the machining program is executed, a machining command for driving the punch head is given to the punch press machine and the specified machining is repeatedly executed on the work in sequence while sequentially moving the work table on which a work is fixed. This type of program for implementing such functions is referred to as a pattern type of program.
However, the operation of the punch press machine may be stopped while the NC unit which controls the punch press machine is executing the program for multiple product machining and giving machining commands to the punch press machine. The operation of the punch press machine is stopped when, for example, drilling is not made as designed because a tool mounted on the punch head is broken. In such a case, it has been difficult for the conventional NC unit having a program restarting function to restart the command program from halfway.
In other words, for restarting machining of a work from halfway, the operator should identify the stop position and change the program in accordance with the above stop position. However, since the multiple product machining program is programmed in a pattern mode having a repetitive loop as described above, a program in the loop must be modified to restart the operation from halfway of the repetitive loop and such modification not only requires a skilled operation but also is a complicated work accompanying the entry of a great deal of data. Accordingly, it takes a lot of time to restart the operation and the operation efficiency of the punch press machine will deteriorate.
In some cases, the machining program is restarted for a new work without changing the program, attaching an importance to the operation efficiency of the punch press machine. In such cases, there has been a problem that all materials in process would be wasted.